The long term aim of this project is to understand the physiological control of cytokine signaling pathways that mediate cellular proliferation and differentiation. These pathways are activated inappropriately by cytokines or are otherwise subverted in a wide range of diseases including cancer, autoimmunity and inflammatory and infectious diseases. Previous work funded under this grant using functional genetic screens identified a family of critical negative regulators of cytokine signaling called the supressors of cytokine signaling (SOCS). Gene deletion experiments revealed that SOCS-1 is an essential molecule required to keep in check the potentially lethal effects of endogenous interferon-gamma and biochemical experiments revealed that SOCS-1 interacts with activated JAK kinases and inhibits their activity. The aims of the present project are: (1) to analyse in gene-deleted mice the unique and redundant physiological roles of three different SOCS proteins (SOCS-1 to - 3). In the case of a lethal phenotype (as for SOCS-1 -/- and SOCS-3-/- mice) tissue-specific, conditional gene deletions or crosses to cytokine-deleted mice will be performed to identify the site of action of the defect. (2) to determine the specificity of different SOCS proteins in inhibiting the actions of different cytokines by performing genetic crosses of mice with different SOCS or cytokine deletions; and (3) to identify the physiological molecular targets that interact with SOCS proteins to mediate their inhibitory effects and determine the biochemical and biological consequences of each interaction. Interacting proteins will be identified by co-association, direct binding and definition of peptide specificity recognition and the quantitative binding affinity of each interaction determined. All analyses based on in vitro biological effects or biochemical interactions will be verified in the whole animal by generating appropriate gene-deleted or -modified mice (e.g. producing a SOCS protein with only one interaction domain deleted) and by appropriate genetic crosses (e.g with mice in which the proposed interaction partner has been deleted). These studies are expected to define physiologically-validated therapeutic targets for the design of molecules that should have therapeutic value in treating diseases associated with the exogenous or endogenous activation of cytokine signaling pathways, especially inflammation, leukemias and other cancers.